Probing topological phase transitions using high-harmonic generation Heide, Christian Kobayashi, Yuki Baykusheva, Denitsa Rangelova Jain, Deepti Sobota, Jonathan A. Hashimoto, Makoto Kirchmann, Patrick S. Oh, Seongshik Heinz, Tony F. Reis, David A. Ghimire, Shambhu Atomic and Molecular Physics and Optics Electronic Optical and Magnetic Materials The prediction and realization of topological insulators have sparked great interest in experimental approaches to the classification of materials1,2,3. The phase transition between non-trivial and trivial topological states is important, not only for basic materials science but also for next-generation technology, such as dissipation-free electronics4. It is therefore crucial to develop advanced probes that are suitable for a wide range of samples and environments. Here we demonstrate that circularly polarized laser-field-driven high-harmonic generation is distinctly sensitive to the non-trivial and trivial topological phases in the prototypical three-dimensional topological insulator bismuth selenide5. The phase transition is chemically initiated by reducing the spin–orbit interaction strength through the substitution of bismuth with indium atoms6,7. We find strikingly different high-harmonic responses of trivial and non-trivial topological surface states that manifest themselves as a conversion efficiency and elliptical dichroism that depend both on the driving laser ellipticity and the crystal orientation. The origins of the anomalous high-harmonic response are corroborated by calculations using the semiconductor optical Bloch equations with pairs of surface and bulk bands. As a purely optical approach, this method offers sensitivity to the electronic structure of the material, including its nonlinear response, and is compatible with a wide range of samples and sample environments. Springer Nature 2022 info:eu-repo/semantics/article doc-type:article text http://purl.org/coar/resource_type/c_2df8fbb1 https://research-explorer.ista.ac.at/record/13991 Heide C, Kobayashi Y, Baykusheva DR, et al. Probing topological phase transitions using high-harmonic generation. <i>Nature Photonics</i>. 2022;16(9):620-624. doi:<a href="https://doi.org/10.1038/s41566-022-01050-7">10.1038/s41566-022-01050-7</a> eng info:eu-repo/semantics/altIdentifier/doi/10.1038/s41566-022-01050-7 info:eu-repo/semantics/altIdentifier/issn/1749-4885 info:eu-repo/semantics/altIdentifier/e-issn/1749-4893 info:eu-repo/semantics/closedAccess